HEDRICK SMITH, FRONTLINE:[voice-over] The story of the September 11th attack on America begins in this three-room apartment in a working-class district of Hamburg, Germany. This was the hub of Usama bin Laden's Hamburg cell, the place where the key hijackers gathered and hatched their deadly plot.

This was the home of the cell's ringleader, Mohamed Atta. The son of a middle-class Egyptian lawyer, he would pilot the first plane, American Airlines flight 11, into the World Trade Center. Following Atta like a shadow was Marwin al-Shehhi, the shy, religious son of an imam from the Persian Gulf. He would fly the second plane into the Trade Center. The third key figure was Ziad Jarrah, an only son of a Lebanese merchant family, a charmer with a zest for life. He would fly United flight 93 to its fiery crash in Pennsylvania.

I came to Hamburg to try to understand these men. What motivated them? How did their cell operate? How did they elude all our intelligence agencies? I found a shadowy story full of passionate commitment and canny deception, a story of iron discipline, arrogant mistakes and sheer luck.

Hamburg is a mecca for ambitious young Arabs. They come here to learn from the West and go home to modernize their own countries. In 1992, a young Egyptian architectural student arrived in this industrial center and enrolled at a technical university to do graduate study in urban planning. Immediately, the serious 24-year-old Mohamed Atta impressed and intrigued his professor.

Prof. DITTMAR MACHULE, Academic Adviser: He was not like young students- looking, laughing. I was rather interested in his development. He was polite. He was somewhat reserved. He was very intelligent in discussing. And argumenting and the special thing, he was very, very religious.

HEDRICK SMITH: Shortly after his arrival in Hamburg, Mohamed Atta, already a qualified architect, took a part-time job with a local design firm.

JORG LEWIN, Architectural Designer: When he was four or five weeks in our office, he asked us if its possible to pray in the office. And we said "Why not? No problem. You can go to corner. And if you want, you can pray."

HEDRICK SMITH: Fellow students who became his friends saw Atta as a workaholic with no taste or time for pleasure.

VOLKER HAUTH, Fellow Student: He was not interested in cars. And I think he was not really interested in girls, like Western young men are.

HEDRICK SMITH: The professor who knew him best always sensed an inner struggle, a schism, within Mohamed Atta.

Prof. DITTMAR MACHULE: How we can imagine that a man like Mohamed could be able to do what he should have done? I can only explain it that there are these two parts of a coin, two parts of a human being. And there's the dark, the bad, the awful part, and the other part, the normal. And he wasn't able to cover this dark part. He went to pray, and this gave him force to be what he was, to be two people.

HEDRICK SMITH: Even in Cairo, there are no easy answers to why a middle- class college graduate with a bright future should have become a killer. But there were signs of conflict within the family, which evidently affected Atta as a child. His father was a successful lawyer, a domineering figure and a strict disciplinarian who sent Mohamed to study in Germany because he had great ambitions for his only son.

MOHAMED AL-AMIR ATTA, Sr., Father:[through interpreter] He was not ordinary. He was exceptional. He was a gentle person, very shy, unassuming and highly sensitive.

HEDRICK SMITH: Atta's mother doted on him, so much that his father considered Mohamed a "mommy's boy." Atta felt overshadowed by his two older sisters, both successful graduates. Friends saw him as soft and immature.

MOHAMMED MUKHTER, Friend: Mohamed, I remember him completely in the form of child. He has child- child feelings, innocent, virgin.

HEDRICK SMITH: When he received his first degree from Cairo University, he seemed purposeful and happy, but reclusive.

MOHAMMED MUKHTER: Really, he's a very delicate person. Sorry. I'm going to cry about him. He was a very nice person. Really!

HEDRICK SMITH: But as Atta and his generation reached their early 20s, they were exposed to the angry passions and resentments swirling through Cairo-

PROTESTERS:[subtitles] God is great! Down with America!

HEDRICK SMITH: -anger at a ruling elite seen as corrupt and undemocratic. Anger, too, at America for supporting Egypt's rulers and being Israel's strongest ally. Atta heard echoes of this anger from his own father, which the father still voices to this day.

MOHAMED AL-AMIR ATTA, Sr.:[through interpreter] The pulse of the Arab streets say that America is a filthy thug! America is a filthy thug who wants to hit the Islamic nations!

HEDRICK SMITH: Two friends from the university in Hamburg saw a similar political hardening in Atta when they went to Cairo with him in 1996 on a joint study project. Atta was openly angry with the Egyptian government for exploiting its poor and at the United States for its policies toward the Islamic world.

RALPH BODENSTEIN, Fellow Student: These politics were- were always in the interests actually of the U.S., and he was very critical of that, actually. I could sense this sort of frustration and rage, especially about what was going on in the world, in the biased politics against Muslims, as he would have regarded it.

HEDRICK SMITH: Atta's rage and frustration were fueled by the government crackdown on Islamic extremists. Atta told his friends he was afraid of being imprisoned himself because he might be linked to Islamic fundamentalists in his engineering school at Cairo University.

VOLKER HAUTH, Fellow Student: Mohamed gave the impression of being sad and kind of being depressive about his individual perspective and about the political situation because he didn't know how to remain in Egypt without being criminalized, because he wanted to live in Egypt.

HEDRICK SMITH: So at 28, Mohamed Atta saw himself an outsider in his own country, as a man with no future there. Returning to Hamburg, with a permissive culture that is so offensive to devout Muslims, Atta felt even more the alien in an alien land.

VOLKER HAUTH: He was very much an Egyptian. The German world, the European world was a strange world for him, not for his intellect, but for his culture and for his religious beliefs.

HEDRICK SMITH: So Atta began searching for a refuge, where he could express his religious views and meet Arab soulmates. He found what he was looking for in a non-descript building in a rough part of town, a place where young men from many Arab countries congregate.

Only a small sign indicates the existence of the Al Quds mosque. Most mosques don't espouse extreme Islamic views, but sometimes these can become meeting places for radical elements. When our cameras visited the mosque, they weren't welcome.

HERBERT MUELLER, Islamic Expert, German Intelligence:[through interpreter] A mosque is definitely not simply a place for praying. There might be small groups who are the representatives, the activists, of a radical fundamentalist Islam, one with terrorist aims. And these would be the ones to seek out capable persons from among their fellow believers.

HEDRICK SMITH: Mohamed Atta grew a beard, a symbol of religious devotion and sometimes a sign of radicalization. Investigators believe Al Quds Mosque was where Atta was radicalized, either through meeting extremists or direct recruitment by an agent of bin Laden.

Mohamed Atta wrote out a will, declaring his willingness to make the supreme sacrifice. It was witnessed by some of his friends at the mosque. He urged them not to be sad when he died.

READER: I don't want anyone to weep or cry or rip their clothes or slap their face because that is an ignorant thing to do.

HEDRICK SMITH: He ordered that no women should attend his funeral, a sign of his growing obsession with sexual purity.

[www.pbs.org: Read Atta's will]

READER: The person who will wash my body near my genitals must wear gloves so he won't touch my genitals.

HERBERT MUELLER:[through interpreter] It could have been a psychological preparation carried out by a person committing suicide, against the background of a mujahid, a warrior following the way of the God.

HEDRICK SMITH: The Al Quds mosque is where investigators believe Mohamed Atta met two other young Arabs who would become members of the Hamburg terror cell and pilots on September 11th, Ziad Jarrah and Marwan Al Shehhi.

Marwan Al Shehhi had come to Germany from the United Arab Emirates, known for the oil and banking wealth of Abu Dhabi and Dubai. But we know little about the pilot who was born in the poorest and most conservative emirate, Rash al-Khainah. We do know al-Shehhi was very close to his father, a local imam, whom he often followed to the mosque. He sometimes allowed the boy to make the call to prayer.

SHEIK ABDULLAH BIN ZAYED, Minister of Information, UAE: Marwan was a very normal individual. He was in his early 20s. He lost his father a year ago, and he got a scholarship to study in Germany, which paid him quite well.

HEDRICK SMITH: But after his father died, al-Shehhi became estranged from his family.

SHEIK ABDULLAH BIN ZAYED: He was a normal individual till he went to Germany. It seems when he went to Germany, he met these radical fanatics who have a very different understanding of Islam than we do.

HEDRICK SMITH: In Hamburg, one of the young men he met was Ziad Jarrah, the man who would pilot United flight 93, which crashed in Pennsylvania. Perhaps the most unlikely terrorist of all, Jarrah remains an enigma to this day.

He came from the Bekaa Valley in Lebanon. Like al-Shehhi, he lived next a mosque, but was hardly religious. His middle-class, Westernized family wanted the best for their only son, so they sent him to the best schools, even though they were Catholic schools steeped in Catholic doctrine, with instruction taught in French.

JAMAL JARRAH, Uncle: He went to the best schools in Lebanon. He was raised gently, as he is the only boy in the family. He is a kind of lovely boy to his parents, to his family, and always he likes to have fun.

HEDRICK SMITH: As a young man heading to university in Germany, his dream was to become a pilot. Once in Hamburg, Jarrah rented a room from a German landlady in a genteel suburb.

ROSEMARY CANAL, Landlady:[through interpreter] Yes, he was a nice young man, really, and we got on well. He used to bring me back presents, too, when he went home. Sometimes we would chat when I was having tea, and I would invite him to have a cup of tea with me.

HEDRICK SMITH: Jarrah began dating and then living with a Turkish-German girl, another secular Muslim, who was about to start medical school.

JAMAL JARRAH: He was living with his girlfriend. We loved the girl. She's very nice. She loved Ziad. And this is against radical Muslim's thinking and behavior and acceptance.

HEDRICK SMITH: According to German intelligence, Islamic fundamentalists often try to recruit young Arabs with technical education. In Hamburg, Jarrah began a college course in aircraft engineering. At first, he seemed his usual self, going out for beers with his classmates. But after a few months, his landlady noticed small changes that suggested a new commitment to Islam. She painted a portrait of his new look.

ROSEMARY CANAL:[through interpreter] You didn't notice anything at first. Later on, he kept small prayer mat in his room, and he grew a beard. And I poked fun at him because of it.

HEDRICK SMITH: Ziad Jarrah had started spending time at the Al Quds mosque with al-Shehhi and Atta. Now their lives were about to take a radical turn. In the summer of 1997, Atta disappeared from his university for over a year. He gave only a vague explanation.

Prof. DITTMAR MACHULE, Academic Adviser: He told me that he was absent because there had been problems in his family.

HEDRICK SMITH: Western intelligence has established that Atta traveled to Afghanistan, to the camps of Al Qaeda. He was followed by al Shehhi and Jarrah. In bin Laden's camps, recruits were trained in the violence of terror, using weapons and explosives, and how to run covert operations in the West. Here it's believed Mohamed Atta revealed leadership potential and was singled out for special training.

It was while Atta was in the camps Usama bin Laden made his most menacing threat against the United States. He declared all Americans to be legitimate targets of terror.

USAMA BIN LADEN:[through translator] We do not have to differentiate between civilians and military. As far as we are concerned, they are all targets.

HEDRICK SMITH: Atta was also in the camps when bin Laden's men bombed two U.S. embassies in East Africa, killing 224 civilians, most of them African. Twelve Americans died. The U.S. struck back but failed to kill bin Laden. Emboldened, he drew even more young men to his cause.

When Mohamed Atta reappeared at his Hamburg university in October, 1998, he was a changed man.

Prof. DITTMAR MACHULE: He looked more serious. He didn't smile as much as before. He worked hard.

HEDRICK SMITH: Atta's university studies took on a new urgency, but he had another, even more urgent agenda. He'd become an operational commander for bin Laden, preparing a big attack. Following the blueprint set out in Al Qaeda's manual, he formed a terror cell. He found an apartment, number 54 Marienstrasse. This apartment became the meeting place for at least six Arab students.

Mohamed Atta, Al Shehhi and Jarrah would become three of the pilots on September 11th. Another roommate, Ramzi Binalshibh, was to have been the fourth pilot, the FBI says.

Atta's German landlord found nothing suspicious about the Arab students who lived in the apartment. They were bearded, but smartly dressed. They seemed respectable. They made only one request, for high-speed phone lines for their two computers.

LANDLORD: We have no problems. We got the rent perfectly. We have no problems with the guys.

HEDRICK SMITH: That winter, German intelligence missed an opportunity to detect the cell and spot their plot. In the aftermath of the Nairobi bombing, the Germans were carrying out surveillance on one of Atta's roommates, who was suspected of having links to bin Laden The activities at the Marienstrasse apartment could have yielded valuable clues about the Hamburg cell's plans. But the Germans didn't grasp what they had stumbled upon because they never focused on Mohamed Atta or the other future pilots.

[www.pbs.org: Chronology of the terror operation]

Meanwhile, the cell stepped up its preparations. Late in 1999, Atta, Al Shehhi and Jarrah all reported their passports stolen. The old ones bore the evidence that they had all been to bin Laden's camps. They needed new ones, to wipe the record clean.

SHEIK ABDULLAH BIN ZAYED, Minister of Information, UAE: So they can get a visa to the U.S. without them knowing that they've been to Afghanistan, Pakistan.

HEDRICK SMITH: Mohamed Atta's life in Germany was drawing to a close. After nine years of study, he submitted his master's thesis on urban renewal in an ancient Islamic city. It earned the highest possible grade.

Prof. DITTMAR MACHULE, Academic Adviser: He got 1.0, and that's the best I can give him. He was one of our best students. He was one of our very best students.

HEDRICK SMITH: On the thesis, Atta had written a dedication that only now seems ominous.

Prof. DITTMAR MACHULE: It says, "Speak, my prayers and my sacrifice and my life and my death belong to Allah, the Lord of the worlds." "My life and my death."

HEDRICK SMITH: In the summer of 2000, the men who would pilot three of the hijacked planes arrived separately in America on tourist visas from Berlin. It was 15 months before September 11th. They quickly made their way to sunny Florida, to capitalize on the good weather to learn how to fly as fast as possible.

Mohamed Atta and Marwan Al Shehhi enrolled at Huffman Aviation. As foreigners, they needed student visas to take lessons. But with just tourist visas, they were accepted anyway- no advance booking and no questions asked.

RUDI DEKKERS, Huffman Aviation: Atta and al-Shehhi walked into the front door. And then we have no obligation whatsoever to do any background checks or passports or ID. They need to ID themselves the moment they're doing a test for the license.

HEDRICK SMITH: Huffman had a lot of foreign students, but instructors found these two Arabs unusual. They didn't mix with other students, and they didn't show any of the typical newcomer's enthusiasm for flying.

[on camera Was it difficult to give Atta instructions? People say he was kind of arrogant, didn't want to take instructions.

THIERRY LEKLOU, Flight Instructor: Yeah, that's- that may be one part of his character was he wasn't very patient with instructions. He was just mostly want to fly the airplane, maybe with no passion, and no patience, either.

HEDRICK SMITH:[voice-over] Several of Atta's fellow students considered him prickly and aloof.

ANNE GREAVES, Student Pilot: What struck me particularly about Mohamed Atta was his eyes. They were immovable. They were extremely cold. And that never wavered. I can remember Atta very well, striding towards the aircraft as though it was a means to an end, rather than pleasure or enjoyment, something you've always wanted to do. That wasn't there. That sparkle just was not there.

HEDRICK SMITH: One day, Anne Greaves was struck by bizarre celebrations coming from the computer room, where the two men had been on the Internet.

ANNE GREAVES: Mohamed Atta and Al Shehhi were hugging each other with joy and almost dancing in the room. And I thought this was extraordinary because I thought, "Wow. There's some life in them, then, after all." It must have been some time towards the middle end of October.

HEDRICK SMITH: On October 12th, the USS Cole, an American destroyer, had been attacked by Al Qaeda suicide bombers off Yemen. Seventeen U.S. seamen were killed. In Afghanistan, bin Laden celebrated and so, apparently, did Atta and Al Shehhi.

But they also had cause to worry. That fall, Atta and Jarrah tried to enroll another member of the Hamburg cell in flight school. He was living in the Marienstrasse apartment. His name: Ramzi Bin Alshibh, whom the FBI thinks was meant to be the fourth hijacker pilot. But he was from Yemen, considered a hotbed of terrorism. So the U.S. denied him a visa four times.

Yet strangely, no one raised a question about his two friends from Hamburg who wanted Bin Alshibh to join them in flight school. No one asked questions, either, about how Atta and al Shehhi, with no visible means of support, were able to pay for expensive flight lessons.

The two men had set up accounts at SunTrust Bank of Florida. The FBI now says Atta and Al-Shehhi were being fed streams of money from abroad, eventually more than $100,000 each. Most of the money was coming from Dubai, one of the world's busiest and most anonymous banking centers. The bank transfers were broken up into smaller amounts to avoid triggering alarms in the international banking system. Investigators believe the money was coming from this man, Mustafa Ahmed al Hawsawi, suspected of being bin Laden's main paymaster.

We asked the Senate Intelligence Committee chairman, shouldn't questions have been raised about the bank transfers?

Sen. BOB GRAHAM (D), Florida: The answer is yes, but let's put this in perspective. The September 11th event is estimated to have cost about $500,000. In a banking system where every day there are trillions of dollars turned over, it is not surprising that that amount of money, particularly over a period of time, could have been lost in the system.

HEDRICK SMITH: But what was surprising, yet still aroused no suspicion, was a trip Atta and Al Shehi made to a jet aircraft simulator. They told the company they wanted to join an airline and needed to familiarize themselves with an airliner cockpit.

[on camera] What do you think they took away from their experience here?

HENRY GEORGE, Flight Simulator Instructor: What they wanted and what we gave them. They wanted an exposure to a jet aircraft. They came from flying a small airplane that flew maybe at 100 miles per hour. The aircraft that we were simulating was flying at 300 miles per hour. So it was a faster airplane, requires a little bit faster train of thought.

My feeling is - you know, looking back, of course - is that it was just- they were doing a job. They were preparing to perform a job. They needed to prepare to- for that task, and they were simply doing it.

HEDRICK SMITH: In December, 2000, Atta and Al-Shehhi finished their flight training in Florida and began to travel back across the Atlantic, evidently to reconnect with Al Qaeda. In January, the third pilot, Ziad Jarrah, flew into Dubai. Under questioning, Jarrah told local authorities that for two months he had been in Pakistan, and, they presumed, in Afghanistan, in bin Laden's camps.

Jarrah said he was a pilot headed back to the United States. He had a valid U.S. visa, and so Dubai let him go. Dubai authorities say they informed the U.S., but the CIA says the U.S. was not told about this trip until after September 11th.

Jarrah then went to Beirut for a family wedding. His parents knew he'd been to America learning how to fly, but didn't know he was a follower of bin Laden.

JAMAL JARRAH, Uncle: He was behaving in a very modern way- dancing, socializing with everybody at the party. He has a lot of friends, and everything was normal and OK. He doesn't have any sign or signal that you might think about that he changed his way of behaviour, or his thinking, even,

HEDRICK SMITH: Mohamed Atta, too, slipped through the fingers of American immigration that January when he came back to Miami from a trip to Spain. An alert immigration officer saw that his tourist visa had expired and that he'd violated its terms by taking flight lessons. He was held for questioning.

But Atta knew how the system worked, and he talked them into granting him an extension for eight months.

Sen. BOB GRAHAM: The fact that Atta was able to get a visa, in the first place, and then to abuse it by leaving the country and coming back even though it had expired, is an indication of the atrocious way our INS operates.

HEDRICK SMITH: Safely back in America, Atta and his lieutenants moved to the beachfront towns north of Miami. And the second wave of the hijack team began to arrive from Saudi Arabia.

These Saudi foot soldiers, in the lingo of the FBI, were hand-picked martyrs to the cause, trained in Al Queda's camps. They were conservative young Muslims who spoke little English. They were invisible to U.S. intelligence.

ROBERT BLITZER, FBI Domestic Terrorism Chief, '96-'98: The 19 hijackers were clean. They weren't criminals. You know, there were no- no significant warning signs in their backgrounds that would have focused law enforcement or the immigration people on them specifically. And you know, even if one or two of them had been focused on significantly, again, in checking them out, you would have found nothing. So you know, you had really nothing to go on. And so they're here.

HEDRICK SMITH: They blended unobtrusively into the ethnic mix of south Florida and began to operate by Al Qaeda's manual. It says, "Keep fit for the mission," and so they worked out in local gyms.

[www.pbs.org: Study al Qaeda's manual]

HEDRICK SMITH: When one of the pilots, Ziad Jarrah, applied for gym membership, he said he would need it only until September. He told master trainer Bert Rodriguez he wanted personal lessons in martial arts.

BERT RODRIGUEZ: The course that he wanted to train is called Close Quarter Aggressive and Defensive Tactics. It entails everything from grappling to choking to striking to knives to guns. This is what he wanted to learn.

He said that he traveled a lot, that he'd been to a lot of martial arts schools, but that he wanted to learn something that was going to be very effective in real-life situations.

HEDRICK SMITH: Mohamed Atta was constantly on the move. His job was to coach the hijackers in how to blend into America: Don't pray in public. Shave your beard. They followed Al Qaeda tradecraft meticulously, to be unobtrusive, using money orders, not banks, to transfer cash.

BRAD WARRICK, Warrick Rent-A-Car: A lot of criminals do come here, simply because, you know, we're a little guy out of the way, and a lot less conspicuous than going to a big place.

HEDRICK SMITH: For credit cards and airline tickets, they found blind, anonymous addresses. For communications, they intentionally went to busy public places.

LEW SCHILIRO, FBI Assistant Director, '98-'00: When you look at the ability to walk into a Kinko's and get on a computer and communicate via email for a short period of time and walk away from it, and perhaps have the ability to erase that communication from ever having occurred- those techniques or that tradecraft was critical, in my estimation, to the events of September 11th.

HEDRICK SMITH: One cardinal rule of Al Qaeda is don't draw the attention of authorities. But the hijackers did make mistakes. Ironically, it was Atta himself who made a mistake that might have thrown the whole operation into jeopardy. At a routine road check, he was pulled over and cited for driving without a license. He was ordered to show up in court the following month, but he never appeared and was never pursued.

Even more serious was the near-disaster that Atta and al-Shehhi ran into right after earning their commercial pilot licenses. They flew a small plane from Huffman Aviation to Miami International Airport and had a mechanical breakdown on a busy taxiway

DAN PURSELL, Former Chief Instructor, Huffman Aviation: We got a phone call, and it was Mr Atta. He said the airplane had stalled, the engine would not keep running while he was on the ground. And I was kind of concerned about that. First thing I asked him was did he have fuel, proper tank, ran through a checklist with him. I suggested that he get the airplane towed and have it evaluated in the morning.

HEDRICK SMITH: What Atta had failed to mention was that they had left the plane on the taxiway. An angry FAA official called Huffman.

DAN PURSELL: He said they abandoned the airplane. And he didn't know all the facts. He was kind of agitated. You know, "You have an airplane down here. The students abandoned it. And we need to know what's going on."

HEDRICK SMITH: The FAA was upset about the plane, but apparently didn't bother to investigate the pilots further.

Sen. BOB GRAHAM: As a licensed pilot, I find this to be a very bizarre and inexplicable situation. The fact that that incident didn't cause some bells to ring both at the airport in Miami, the FAA law enforcement agencies, is hard to believe. And if those bells had rung, it might have led to a series of investigations which would have peeled back this plot which was beginning to develop, and potentially could have avoided September 11th.

HEDRICK SMITH: In the months leading up to the attack, Atta was a whirlwind of travel. He made several more trips to Europe. No one knows for sure what he was doing, but U.S. intelligence believes he was mainly making contacts with the Al Qaeda network. He traveled to Prague, where he was observed in a still unexplained meeting with the station chief of Iraqi intelligence. On another trip, to Madrid, the FBI believes Atta made contact with a major Al Qaeda cell that investigators have now linked to Atta's U.S. oeprations.

And Atta returned to Hamburg, where some members of his cell were still living. On one visit, he also had a rendezvous with three Arabs who had evidently come from Afghanistan to see him. In southern Germany, the three Arabs had hired a local taxi driver and asked him to take them 400 miles north to Hamburg.

The driver began chatting with the men. They passed a road accident where people lay injured, and the man in the front seat said that he'd seen plenty of dead bodies in Afghanistan.

TAXI DRIVER:[through interpreter] So I asked him if he had been a soldier in Afghanistan. He said, "Yes." [in English] He [unintelligible] that he was a soldier in Afghanistan.

HEDRICK SMITH: They told the taxi driver to take them to Hamburg railroad station, where a friend would be waiting and would pay the fare.

TAXI DRIVER:[through interpreter] They greeted each other with a big bear hug when they met. And then he walked over to me at the car, but he didn't smile or anything.

HEDRICK SMITH: After September 11th, the taxi driver immediately recognized the unsmiling man at the station.

TAXI DRIVER:[through interpreter] I saw the picture on television of Atta, and I knew that he had been the man who had paid for the taxi here in Hamburg.

LEW SHILIRO: When he traveled, he probably met either directly or indirectly members of Al Qaeda. He probably received instructions from the hierarchy associated with Usama bin Laden, from his emissaries. He probably received instructions on who to contact for financial and logistical support. Those trips were critical, in my mind, to the success that they unfortunately had on September 11th.

HEDRICK SMITH: On July 4th, two Saudi members of the hijacking team flew into Newark from abroad. They'd previously been training on the West Coast. They joined other Saudis who had rented an apartment above a mini-mart in the poor part of Paterson, New Jersey.

For several months, the landlord told me, four Saudis lived in the apartment without any furnishings, just a couple of bare mattresses on the floor. Several times Atta came from Florida to see them, and was spotted on the streets of Paterson.

Like the group in Florida, they all had clean records, except for one of the new arrivals, Khalid al-Midhar. That January, the CIA had learned that al-Midhar had been involved in planning the bombing of the USS Cole. But his name wasn't put on an immigration watch list until the end of August, after he was already in the country. Agents would still be searching for al-Midhar and his companion on September 11th.

Sen. BOB GRAHAM: They should have been picked up when they entered the United States. Failed to do so. It also was a breakdown in the hand-off of information from the intelligence services to the domestic law enforcement agencies. Several of these people had been tracked by intelligence until they got inside the United States, and then they were lost in our own country.

HEDRICK SMITH: Perhaps the greatest missed opportunity to foil the plot came just a month before the attack, when a French Moroccan student was jailed in Minnesota. Zacarias Moussaoui aroused suspicion as an inexperienced pilot seeking advanced jet simulator training for a 747 airliner. The flight school alerted the FBI, noting that a fully fueled 747 could be used as a bomb.

Sen. BOB GRAHAM: That was sufficient to cause the operator of the simulator to bring it to the attention of the local FBI, but it wasn't sufficient to cause central FBI to say we need to conduct an aggressive investigation to find out what Mussaoui is up to.

HEDRICK SMITH: After September 11th, the FBI did investigate and discovered Moussaoui had direct links with the Hamburg cell. Ramzi Bin Alshib had sent him $14,000 to pay for the flight lessons. The FBI believes Moussaoui was the original replacement for Bin al-Shibh as the fourth pilot. The FBI now believes Moussaoui was tapped to take Bin al-Shibh's place as the 20th hijacker.

Sen. BOB GRAHAM: If we had done that investigation, it might have been the beginning domino of a whole chain that would have led back to the plot that hit us on September the 11th.

[www.pbs.org: More on the intelligence failure]

HEDRICK SMITH: It was now the month before the attacks. The 19 hijackers were in place, including a fourth pilot, Hani Hanjour. As a fallback, Hanjour had long been in training. By now the targets in New York and Washington had been fixed. The terrorists were rehearsing according to the Al Qaeda blueprint, making test runs on the flights they would hijack. Coldly, they checked out whether security would let their box-cutters pass and how close air crews would let them get to the cockpit.

They discovered Tuesdays had the lightest traffic: more chance of on-time takeoffs, fewer passengers to control. Then they booked seats up front, business class.

Two days before the attacks. Boston's Logan Airport. Mohamed Atta was watching American Airlines flight 11 prepare for departure. A passenger on that flight considered his behavior suspicious.

JANICE SHINEMAN, Passenger: He walked very aggressively, with his head held very high, looking around. When I was actually boarding the airplane, he was standing at the gate counter, writing in a card. Really, the only thing he could see of the airplane was the pilots and the front of the airplane. I thought to myself, "This is very strange," and I was actually afraid.

I actually watched everyone get on the plane because I'd made a promise to myself that if he got on the plane, I would go see the captain because his behavior had been so bizarre.

HEDRICK SMITH: The terror operation was in its final 24 hours. In the United Arab Emirates, bin Laden's mysterious money man appeared again, this time to collect cash sent back from America, $5,000 dollars each from Mohamed Atta and two other hijackers. They were obeying Al Qaeda's rules, returning money to finance future missions. The transfers were received by Mustafa Ahmed.

SHEIK ABDULLAH BIN ZAYED, Minister of Information, UAE: The next day, he disappears to Pakistan. That was the day of the attacks. I wouldn't say more than that. That explains a very strong link.

HEDRICK SMITH: Today, Mustafa Ahmed is a wanted man. So, too, are the three remaining members of the Hamburg cell. They fled just days before the attacks, evidently headed for Afghanistan. They hold the key to many unanswered questions about the September 11th plot.

On the eve of the attack, Mohamed Atta and a young Saudi hijacker drove to Portland, Maine, where they would begin their flight the next day. They checked into the Comfort Inn. Their final hours would be spent wandering around Portland. Various security cameras captured their images. In the photos, Atta appears stern-faced, as always.

A document entitled "The Last Night" would later be found in Atta's luggage, also in a car left by other hijackers and in the wreckage of a plane. It made clear that death lay ahead.

READER: Make an oath to die. Remind yourself that in this night, you will face many challenges. Obey God. Stand fast.

HEDRICK SMITH: In the early hours of September 11th, Ziad Jarrah called his girlfriend from the airport hotel in Newark. He had told his family he'd be bringing her soon to another family wedding.

JAMAL JARRAH, Uncle: He was planning to pass by Hamburg to see if she can come with him to Lebanon before September 22 to attend the wedding. In fact, he told his father that "I already bought the suit for the wedding of my cousin." Everything was normal and OK. He doesn't have any sign or signal that he changed his way of behavior or his thinking, even.

HEDRICK SMITH: In Portland, Maine, Atta and his companion caught a short flight to Boston. In these last pictures, the men show no sign of what lay ahead.

READER:["The Last Night"] When the hour of reality, the zero hour, approaches, open your heart and welcome death for the sake of God.

BYSTANDER: Holy shit!

HEDRICK SMITH: For Mohamed Atta, the zero hour came at 8:45.

At Boston's Logan airport, another group of hijackers had boarded a plane. Marwan al Shehhi, shadowing Atta to the end, would fly a second jet into the World Trade Center.

READER:["The Last Night"] Remember to pray before reaching the target. After that, God willing, we will meet in paradise.

HEDRICK SMITH: It was 9:05.

As New York burned and a third plane crashed into the Pentagon, the final act of the tragedy was unfolding on board Flight 93 out of Newark.

At 9:25, just as passengers were settling in for a cross-country flight, the hijackers stormed the cockpit, the captain shouting, "Get out of here!" A few minutes later, air traffic control heard another voice on the intercom, thought to be Ziad Jarrah's.

HIJACKER: Hi. This is the captain. We'd like to ask you to remain seated. We have a bomb on board, and we are going to go back to the airport and have our demands, so please remain quiet.

HEDRICK SMITH: Air traffic control asked other planes if they, too, had heard Jarrah's threat.

CONTROLLER: Executive 956, did you understand that transmission?

EXECUTIVE PILOT: Affirmative. He said there was a bomb on board.

CONTROLLER: United 93, I understand you have a bomb on board. Go ahead.

HEDRICK SMITH: They called Flight 93 22 times, all in vain.

CONTROLLER: United niner-three, United niner-three. Do you hear Cleveland?

CONTROLLER: United 93, United 93, Cleveland.

CONTROLLER: United 93, do you still hear Cleveland?

HEDRICK SMITH: At 10:10 flight 93 crashed into the fields near Pittsburgh.

The enduring shock of September 11th is that we did not understand the world we live in, understand that educated young men with bright futures would burn with such hatred that they would die to destroy us. They succeeded by commitment and cunning. We failed from complacency and poor imagination. They caught us by surprise because we did not dream this could happen here.

We can only hope that part of what lies buried beneath the ashes at Ground Zero are America's illusions.

FRONTLINE is a production of WGBH Boston, which is solely responsible for its content.

ANNOUNCER: There's more to explore about this report on our Web site, including a full chronology of the terrorists' movements and activities, more on the intelligence failure of September 11th, excerpts from the terrorists' writings and al Qaeda's manual, and a chance to join the discussion about this FRONTLINE report at PBS on line, pbs.org. Or write and email to frontline@pbs.org, or write to this address. [DEAR FRONTLINE, 125 Western Ave., Boston, MA 02134]

Next time on FRONTLINE: Once upon a time, we bought into a dream.

LISE BUYER, Analyst: A lot of companies that looked like there was no there there were going to the moon.

ANNOUNCER: Were we blinded by our own greed, or did Wall Street take us for a ride?